1. Field of the Invention
The present invention relates generally to image-forming apparatuses and optical scanners, and more particularly to an image-forming apparatus and an optical scanner that employ an LD (laser diode) optical system.
2. Description of the Related Art
Some conventional image-forming apparatuses employ a laser beam to form an image. According to this laser-beam method, the image-forming apparatuses form an image by an optical scanner emitting a laser beam on the scanning surface (or the surface to be scanned) of a photosensitive body (a medium to be scanned).
A description is given below, with reference to FIG. 1, of the operation of forming an image on a photosensitive body by an optical scanner according to the conventional laser-beam method. FIG. 1 is a diagram showing an image-forming operation by a conventional image-forming apparatus employing the laser-beam method.
First, a charging unit (not graphically represented) such as a roller-type contact charger evenly charges a photosensitive body. A laser light source 1101 emits a laser beam to a rotary polygon mirror 1103. The polygon mirror 1103 periodically deflects the laser beam emitted from the laser light source 1101 so that the laser beam is transmitted through an fθ lens 1105 to scan the scanning surface of the photosensitive body repeatedly along a main scanning direction, the photosensitive body moving (rotating) in a sub scanning direction. On the photosensitive body, static electricity is removed from a beam spot or a part onto which the laser beam is emitted, so that an electrostatic pattern (an electrostatic latent image) is formed on the photosensitive body.
A controller (not graphically represented) causes image data in units of pages to be fed line by line (scan by scan) as an image signal (a video signal) to a laser driving circuit. The laser driving circuit outputs the image signal to the laser light source 1101 in synchronization with a pixel clock signal (a writing clock signal) to perform modulation. The pixel clock signal is input via a phase synchronization circuit from a pixel clock generator circuit (not graphically represented) forming a pixel clock generation part and a phase setting part.
Next, a description is given, with reference to FIG. 2, of the relationship between the pixel clock signal and its phase change (phase setting). FIG. 2 is a timing chart showing an example of the relationship between the pixel clock signal and its phase change. Referring to FIG. 2, the pixel clock generator circuit establishes synchronization with a synchronization detection signal input from a synchronization detection sensor, and generates and outputs a pixel clock signal clkw of (b), using a reference clock signal (an original clock signal) clko of (a) n times (four times in FIG. 2) the frequency of the pixel clock signal clkw and toggling the signal level between HIGH (H) and LOW (L) every four clock pulses of clko by count control. The reference clock signal clko is generated from an oscillator not graphically represented.
When the above-described optical scanner writes the electrostatic latent image by forming laser beam spots on the scanning surface, the optical scanner performs control so that the laser beam spots are written or formed at a uniform density.
However, once an environmental variation such as a change in temperature occurs around the fθ lens 1105, the fθ lens 1105 undergoes distortion so that its refractive index is changed. Further, when an environmental variation such as a change in temperature occurs around the laser light source 1101, the wavelength of the laser beam emitted from the laser light source is changed. The fθ lens 1105 refracts the entering laser beam at a predetermined angle in accordance with the wavelength of the entering laser beam. As a result, as shown in FIG. 1, an error may be caused in the angle of refraction of the laser beam entering the fθ lens 1105 so as to cause an error in the writing magnification (optical scanning length) per main scanning period (hereinafter, scanning period) of the laser beam deflected by the polygon mirror 1103, thus affecting an output image. In this case, the phase of the pixel clock signal clkw is shifted by phase changing to correct the writing magnification of the laser beam.
In the above-described optical scanner, the pixel clock generator circuit performs phase control using external pulse trains xpls in order to perform phase changing to shift the phase of the pixel clock signal clkw.
There are two types of external pulse trains xpls employed: an external pulse train xplsp for delaying the phase of the pixel clock signal clkw (indicated by (c) in FIG. 2) and an external pulse train xplsm for advancing the phase of the pixel clock signal clkw (indicated by (d) in FIG. 2).
For instance, in the case of generating the pixel clock signal clkw from the reference clock signal clko, the external pulse train xplsp or xplsm is input to increase or decrease the number of pulses to be counted of the reference clock signal clko so that a pulse of the pixel clock signal clkw, which is normally generated at an interval of eight pulses of the reference clock signal clko, is generated at an interval of nine or seven pulses of the reference clock signal clko. By changing (decreasing or increasing) the number of pulses to be counted of the reference clock signal clko, the frequency of the pixel clock signal clkw is multiplied by 8/7 (advancing control) or 8/9 (delaying control). As a result, it is possible to shift the pixel clock signal clkw after the phase changing. Letting the time of one main scanning line be Tm, this results in Tm−7/8 (advancing control) (indicated by (f) in FIG. 2) or Tm+9/8 (delaying control) (indicated by (e) in FIG. 2). As a result, the effect that the magnification of the line is enlarged or reduced can be produced. Thus, the optical scanner can form an image at a desired position on the photosensitive body irrespective of an environmental variation.
The pixel clock generator circuit includes a pulse generator circuit generating the external pulse trains xplsp and xplsm. The pulse generator circuit generates the external pulse train (hereinafter also referred to simply as pulses) xplsp or xplsm in accordance with a portion of the pixel clock pulse (train) clkw on which portion it is desired to perform phase changing.
Next, a description is given, with reference to FIG. 3, of the generation of the external pulse train xpls (xplsp or xplsm) by the pulse generator circuit. FIG. 3 is a diagram showing a conventional pulse generator circuit.
Referring to FIG. 3, the pulse generator circuit includes comparators 1001 and 1002 and counters 1003 and 1004.
In the pulse generator circuit, an engine CPU (not graphically represented) sets a pulse generation interval (period) prd in the comparator 1001, and sets the number of pulses num in the comparator 1002. The pulse generator circuit operates as follows when the laser beam is deflected by the polygon mirror 1103 to perform a scan in the main scanning direction.
When a clear signal xlclr generated from the synchronization detection signal by a circuit (not graphically represented) is input to the counter 1003, the counter 1003, using the inputting as a reference point, starts a counting operation to count the number of pulses of the pixel clock signal clkw (a count value i), and stops the counting operation when a stop signal is input to the counter 1003 from the comparator 1002.
The comparator 1001 compares the count value i of the counter 1003 and the preset pulse generation interval (hereinafter also referred to as a set value) prd, and generates a pulse (xpls) every time the count value i reaches the set value prd.
The counter 1004 counts the number of pulses xpls generated from the comparator 1001 (a count value j).
The comparator 1002 compares the count value j of the counter 1004 and the preset number of pulses (hereinafter also referred to as a set value) num, and generates the stop signal when the count value j reaches the set value num.
A description is given below, with reference to FIG. 4, of the generation of the external pulse train xpls in the conventional pulse generator circuit. FIG. 4 is a flowchart of the operation of the pulse generator circuit of FIG. 3.
First, in step S1001, when the pulse generator circuit is turned on, the counters 1003 and 1004 reset their respective count values i and j each to “1.”
Thereafter, in step S1002, it is determined whether the clear signal xlclr has been input to the counter 1003. When “NO” in step S1002, the counter 1003 waits until the clear signal xlclr is input thereto. When “YES” in step S1002, after the inputting of the clear signal xlclr, in step S1003, the counter 1003 increments the count value i by one every time a pulse of the pixel clock signal clkw is input thereto.
Then, in step S1004, the comparator 1001 compares the count value i and the set value prd, and determines whether the count value i has reached the set value prd. If the count value i has not reached the set value prd (that is, “NO” in step S1004), the counter 1003 increments the count value i by one. The counter 1003 repeats the operation of step S1003 until the count value i reaches the set value prd.
When the count value i has reached the set value prd (that is, “YES” in step S1004), in step S1005, the comparator 1001 outputs a pulse (xpls). The generated pulse xpls is input to the counter 1003 so that the count value i of the counter 1003 is reset to “1.”
In step S1006, the comparator 1002 compares the count value j of the counter 1004 and the set value num, and determines whether the count value j of the counter 1004 has reached the set value num. If the count value j has not reached the set value num (that is, “NO” in step S1006), in step S1007, the counter 1004 increments the count value j by one when the pulse xpls is input thereto.
Thereafter, the counters 1003 and 1004 and the comparator 1001 repeat the above-described operations. When the count value j reaches the set value num (that is, “YES” in step S1006), the comparator 1002 generates the stop signal. As a result, the pulse generator circuit ends the above-described series of operations, which are hereinafter referred to as a pulse generation operation.
FIG. 5 is a timing chart showing the conventional relationship between the clear signal xlclr and the external pulse train xpls.
Referring to FIG. 5, a pulse train generation part starts to output the external pulse train xpls indicated by (b) after the set value prd passes since the inputting of the clear signal xlclr indicated by (a). At this point, the pulse train generation part outputs as many pulses of a pulse width of one clock pulse clkw of the external pulse train xpls as the number of pulses num at the periods (intervals) prd before the inputting of the next clear signal xlclr.
Alternatively, using a RAM table, a fixed pulse train may be generated from output data obtained by counting up addresses based on a pixel clock signal.
FIG. 6 is a diagram showing the conventional image-forming apparatus with no fθ lens. As shown in FIG. 6, when the image-forming apparatus has no fθ lens, the optical system performs scanning with the laser beam being deflected by the polygon mirror 1103 at equal angles, that is, in a way to draw an arc. Accordingly, when scanning is performed linearly for a line on the photosensitive body surface, the light beam forms beam spots (images) on the photosensitive body surface at varying intervals even if the same interval is assumed for the center and the ends of the line. Further, the distance of beam emission to the photosensitive body surface varies between the center and the ends of the line. As a result, the beam diameter also varies between the center and the ends of the line during the single scan on the photosensitive body surface.
In contrast, the image-forming apparatus of FIG. 1 includes the fθ lens 1105 (and another lens group) in order to control the variations in beam spot interval and beam diameter caused in the image-forming apparatus of FIG. 6.
According to the image-forming apparatus of FIG. 1, the refractive index is controlled so as to absorb the difference in magnification between the center and each end of a single scanning line on the photosensitive body.
Such prior art technologies for controlling the magnification of an image in the main scanning direction as described above are disclosed in, for instance, Japanese Laid-Open Patent Applications No. 2000-141754 (JP 2000-141754) and No. 2001-318327 (JP 2001-318327). According to JP 2000-141754, in an image-forming apparatus where a light source is driven according to an image signal based on a writing clock signal, a scanning part causes a light beam emitted from the light source to perform scanning in the main scanning direction on a photosensitive body moving in the sub scanning direction so that an image is written on the photosensitive body, and the image on the photosensitive body is transferred onto transfer paper, the magnification of the image in the main scanning direction is corrected based on the size of the transfer paper in the main scanning direction.
According to JP 2001-318327, an optical beam scanner divides a scanning region on a scanning surface in two in the main scanning direction, and performs scanning with two imaging optical systems.
FIG. 7 is a graph showing beam emission positions on a photosensitive body surface and magnification deviations according to a conventional image-forming apparatus using an fθ lens as shown in FIG. 1.
In the conventional image-forming apparatus, the characteristic of the fθ lens is corrected on average. As a result, there is a tendency for magnification to become positive (+) within a predetermined distance from each end toward a center and negative (−) in the vicinity of the center and each end on a single scanning line.
In the case of forming an image by superposing several colors and in an optical system with emphasis on accuracy, such error components cause misregistration between color images, thus degrading image quality.
Even in the case of dividing an image region equally into multiple areas on a single scanning line so that magnification is corrected area by area, corrections are concentrated in particular areas. As a result, an increased number of pulses cannot be accommodated in such areas, thus preventing accurate correction from being performed.